I'm planning on using pre-paid internet cards, since it's cheaper (I live in the Philippines), but a computer shop said that their phone bills were too high, they found out from their bill that they made international calls which they didn't make. They think that through the internet, it was a site or a hidden program that made that international call. Can a firewall prevent this unwanted call or is there another way to prevent this?

A firewall cannot block dialers.

A way to prevent bad things happening to your PC is not to use Internet Explorer and not visit fishy websites (goat porn is bad! ) and use a virus scanner and scan your computer for spyware. And use common sense!

you could try useing a differnt internet browser like netscape or modzilla, (i dont no if they are the same thing or not, but anyway)

also if you are looking for a good and free virus scanner try
http://free.grisoft.com

i have been using it ever since i got rid of crappy norton

so have phunn!